Electric earth transients in geophysical prospecting



My 28, 1940. L. 'w. BLAU ET AL ELECTRIC EARTH TRANSIENTS IN GEOPHYSICALPROSPECTING Original Filed Nov. 15, 1936 rqxmmojBmd u lllll 9K CBM llelectrodes.

Patente'cl May 28, 1940 UNITED STATES ELECTRIC. EARTH TBANSIENTS INGEOPHYSICAL PROSPECTING Ludwig W. Blau and Louis Statham, Houston,

Tex.,

assignors to Standard Oil Development Company, a corporation of DelawareOriginal application November 13, 1936, Serial 110,566. Divided and thisapplication December 30, 1938, Serial No. 248,366-

7 Claims.

The present invention relates to an improvement in the use of electricearth'transients in geophysical prospecting and is a division of theinvention disclosed in our copending application Serial No. 110,566,filed Nov. 13, 1936, now Patent No. 2,160,824 of June 6, 1939.

In electric transient prospecting a transient volumes of the earth, torecord the transients so produced in opposite phase at a singlereceiving station, and to observe'the diiference between the transients.This is described in the co-pending application of Louis Statham, No.54,179, filed December 13, 1935, now Patent No. 2,113,749 of 26 April 4,1938, entitled Method and apparatus for comparing electrical transients.

According to the embodiment of our original invention, to which ourpresent claims are directed, transient prospecting is simplified by 10-eating the pickup points on equipotential lines along the path of thecurrent. Briefly, in this form of the invention two electricallyconnected electrodes are embedded in the ground at a selected distancefrom each other ranging from a 95 few hundred feet to several thousandfeet, two

thousand feet being a convenient spacing. Di-

- rect currentE caused to flow in the circuit of the electrodes untilsteady state conditions are attained; then, by the use of a pair ofelectrically connected probe electrodes, equipotential lines are locatedalong the path of the current. This is usually done by arranging theprobe electrodes on either side of a line connecting the chargingelectrodes and moving the probe electrodes until there is no potentialdifference between them.

' When an equipotential line is located, two electrically connectedpickup electrodes, in the circuit of which is included an oscillogr-aph,or any other wave tracing means, are arranged on the equipotential lineat a distance from each other ranging from a few hundred feet to severalhundred feet, a suitable distance being about five hundred feet,preferably with a pickup electrode on either side of a line connectingthe charging A transient is then set up between the charging electrodeseither by breaking the circuit already established, by changing thevoltage in said circuit, or by breaking the circuit and, after a pauseof a few minutes, again closing'it.

This embodiment of our invention will be bet- 5 ter understood from thefollowing detailed description of the accompanying drawing in which thesingle figure is a plan view showing one arrangement of apparatus whichmay be embodied and in which current lines and equipotential lines 10are graphically illustrated.

Referring to the drawing in detail, a direct electric current is passedthrough the earth 'by. means of battery I25 and switch I26 which areconnected through a line I2'I with current elecl5 trodes I28 and I29embedded in the ground. Reference numerals I40 to I46 inclusiverepresent equipotential lines which are created at the earth's surfaceby a diiference of potential between the energization electrodes I28 andI29 20 if the subjacent earth within significant proximity of theseelectrodes is of uniform conductivity. These equipotentials representthe intersection of the earth's surface with equipotential surfaces. Thecurrent lines are designated I49 5 and the direction of current flow bythe arrows I49. 'Potential electrodes I60 and I6I are embedded in theground and are connected by lines I62 and I63 to an amplifier I64 andoscillograph I65. go

In the embodiment illustrated, the potential electrodes I60 and I6I aredisposed on the median potential line I43 but may be placed On anyequipotential line as shown by the dot and dash line connection toelectrodes I60 and I6l'. In 5 using this form of the invention, atransient electric' field is observed without the direct component sinceboth potential electrodes are placed on the same steady stateequipotential line. The transient observed in this manner will have the40 same general appearance as the transient differv ence obtained whentwo separate potential circuits are established at other thanequipotential points along the current path and the transients obtainedin said circuits are opposed to each other 45 in the recordingapparatus. The various defleotions ofthe transient from the zero lineindicate, however, when this embodiment of the invention is employed,the tilting of the various beds-o1 which the earth is composed. If thedeflections 5o vanish when the potential electrodes are placedsymmetrically on either side of a straight line connecting the twocurrent electrodes I28 and I29, then the beds are parallel to thesurface of the earth, but, if the deflections do not vanish, u

the beds are not parallel to the surface and the angles of deviationfrom the parallel are proportional :to the magnitude of the deflections.

While the invention has been described as applied to the surface of theearth, it will be understood that it can be carried out in water, underwater, along the sides of clifis, or other vertical structures, inboreholes such as oil wells. or in mines. Also it can be applied to thedetermination of the electrical properties of other materials, such asmetals, semi-conductors, and insulators.

Various changes and alternative arrangements may be made within thescope of the appended claims in which it is our intention to claim thenovelty inherent in the invention as broadly as the prior art permits.

We claim:

1. The method of investigating the properties of matter which comprisespassing a steady state current between spaced electrodes in the matter,locating an equal potential line in said matter due to said steady statecurrent, locating a second pair of spaced electrodes on saidequipotential line, changing the value of said current, and exhibiting avalue indicative of the difference in potential between said receivingelectrodes due to said changing current.

2. The method of investigating the properties of matter which comprisespassing a direct electric current between spaced electrodes in thematter, whereby equipotential lines are set up along the path of thecurrent in the matter, receiving a transient of said current as modifiedby its passage through the matter at spaced potential electrodes locatedon one of said equipotential lines, and exhibiting the difference inpotential between said potential electrodes due to said transient.

3. The method of investigating the properties of matter which comprisespassing a direct current between spaced electrodes in the matter, 10-cating an equipotential line in said matter under steady stateconditions of said current, arranging a pair of spaced electricallyconnected electrodes on said equipotential line, initiating a transientbetween said first mentioned electrodes and observing the difference inpotential between said second mentioned electrodes due to saidtransient.

4. The method of geophysical prospecting I which comprises passing asteady state current between spaced electrodes in the ground in the areato be investigated, locating an equipotential line in the ground due tosaid steady state current, locating a second pair of spaced electrodeson said equipotential line, changing the value of said current andexhibiting a value indicative of the difierence in potential betweensaid receiving electrodes due to said changing current.

5. The method of geophysical prospecting which comprises passing adirect electric current between spaced electrodes in the ground in thearea to be investigated, whereby equipotential lines are set up alongthe path of the current in the ground, receiving a transient of saidcurrent as modified by its passage through the ground at spacedpotential electrodes located on one of said equipotential lines, andexhibiting the difference of potential between said potential electrodesdue to said transient.

6. The method of geophysical prospecting which comprises passing adirect current between spaced electrodes in the ground in the area to beinvestigated, locating an equipotential line in the ground under steadystate conditions of said current, arranging a pair of spacedelectrically connected electrodes on said equipotential line, initiatinga transient between said first mentioned electrodes, and observing thedifference in potential between said second mentioned electrodes due tosaid transient.

7. The method of geophysical prospecting which comprises passing adirect current between spaced electrodes in the ground in the area to beinvestigated, locating the median equipotential line in the groundbetween said electrodes under steady state conditions of said current,arranging a pair of spaced electrically connected electrodes on saidequipotential line symmetrically with respect to said first mentionedelectrodes, initiating a transient between said first mentionedelectrodes and observing the difierence in potential between said secondmentioned electrodes due to said transient.

LUDWIG W. BLAU. LOUIS STATHAM.

